Optical system for mapping signal light onto a detector

ABSTRACT

The invention relates to an optical system that particularly allows an improved detection of signal light propagating from a light source ( 1 ) through a flat glass substrate ( 11 ). SC-modes of this signal light that would normally be totally internally reflected at the backside ( 10 ) of the substrate ( 11 ) are coupled out by a first diffractive optical element DOE ( 21 ). To map all signal light leaving the substrate ( 11 ) onto a single target location ( 51 ), a focusing lens ( 31 ) and a second DOE ( 41 ) are disposed in the optical path behind the substrate ( 11 ). The DOEs ( 21, 41 ) may for example be a ID sinusoidal grating or a 2D blaze grating. The optical system may particularly be applied in an investigation apparatus for detecting multiple spots of a fluorescent sample material.

The invention relates to an optical system and a method for the mappingof signal light from at least one light source onto a target location.

In the WO 02/059583 A1 a detailed analysis is given of the propagationof signal light from a luminescent sample through a glass substrate. Theanalysis shows that a large part of the intensity is contained inso-called “SC-modes” which by definition comprise signal light thatreaches the backside of the glass substrate (i.e. the side opposite tothe sample) under angles of total internal reflection. Signal light ofthe SC-modes is therefore normally lost for detection purposes. In orderto prevent this loss, it is proposed in the WO 02/059583 A1 to disposediffractive optical elements on the backside of the glass substratewhich couple light of the SC-modes out of the glass substrate bydiffraction. A problem with this approach is however that the signallight leaving the glass substrate is spread over a large range of angleswhich must be covered by a detector to collect all available signallight. Moreover, the emissions of signal light from different lightsources mix and can therefore not be spatially separated by a detector.

Based on this situation it was an object of the present invention toprovide means for an improved, particularly a spatially resolvedprocessing of signal light.

This object is achieved by an optical system according to an embodimentand a method according to another embodiment. Preferred embodiments aredisclosed in the dependent claims.

According to its first aspect, the invention comprises an optical systemwith an imaging unit for the mapping of light (called “signal light” inthe following for purposes of reference) from at least one light sourceonto a target location, wherein the target location corresponds to theimage of the light source. The light source may for example be aluminescent spot of sample material in a (bio-)chemical investigation ora technical component like an LED. The imaging unit typically focusessignal light onto an image plane according to the principles ofgeometrical optics. It may particularly comprise one or more lenses,wherein the numerical aperture (NA) of the imaging unit (i.e. the lensfacing the light source) is preferably larger than 0.8, most preferablyas large as the index of the medium surrounding said lens. The opticalsystem further comprises the following components:

-   -   a) At least one first diffractive optical element (abbreviated        DOE in the following) which is located “in front of” the imaging        unit with respect to the optical path of the signal light, i.e.        signal light will be diffracted by said first DOE before        entering the imaging unit.    -   b) At least one second DOE that is located “behind” the imaging        unit with respect to the optical path of the signal light, i.e.        signal light has to leave the imaging unit before it can enter        the second DOE. Suitable realizations of the first and second        DOE will be described in more detail in the following with        respect to preferred embodiments of the invention.

An optical system of the aforementioned kind has the advantage toprovide a desired functionality of the first DOE, which may for examplebe a wavelength filter with a pronounced transmission for a smallwavelength regime, while at the same time less desired effects of thefirst DOE can be compensated by the second DOE.

The first DOE and the second DOE may particularly be arranged anddesigned such that the effect which the first DOE has on the path oflight rays passing through it is reversed by the second DOE. With otherwords the optical system as a whole images the input spot in a similarway as if there would be no gratings present and the imaging unit (e.g.lens) would image the input spot while still benefiting from the firstDOE. Thus desired effects of the first DOE on the signal light (e.g. awavelength filtering) are preserved while simultaneously an undisturbedoptical imaging of this light can be achieved.

The first and the second DOE may in principle have a different design(i.e. form and/or dimension). In a preferred embodiment, the first andthe second DOE are however identical in design.

According to another optional embodiment, the first and the second DOEare used in a mirrored arrangement. If the DOEs are identical, too, thesecond DOE may then reverse the effects that the first DOE had on theoptical path of the signal light.

In a preferred realization of the invention, the optical systemcomprises additionally an at least partially transparent substrate witha (curved or flat) backside, wherein signal light from the light sourcecan be coupled into the substrate and wherein at least a part of thissignal light can leave the substrate through the backside. The“backside” is one of the sides of the substrate which is given this namefor reference and based on a view from the light source. The location ofthe light source with respect to the substrate is not restricted in anyway; the light source may particularly be remote from, adjacent to, oreven embedded in the substrate. In many cases the substrate will be asubstantially flat plate made from glass or a transparent polymer.Moreover, the first diffractive optical element DOE is located at thebackside of the substrate and adapted to couple signal light of SC-modesout of the substrate. “SC-modes” comprise by definition signal lightthat would be totally internally reflected at the backside if the firstDOE would not be present. A detailed description of the SC-modes andsuitable realizations of the first DOE can be found in the WO 02/059583A1. The optical system (particularly its imaging unit and the secondDOE) may particularly be designed such that more than 80%, preferablymore than 90%, most preferably all of the signal light of the SC-modesthat was coupled out of the substrate will be directed to the targetlocation.

An optical system of the aforementioned kind has the advantage toprovide a high yield of signal light due to the first DOE that couplesout light which would normally be captured inside the substrate.Furthermore, the spreading of the signal light which is introduced bythe first DOE and which corrupts the normal geometrical imaging of thelight source is reversed by the second DOE such that finally the signallight of the SC-modes (or at least a large part of it) reaches thetarget location. A detector for measuring signal light from the lightsource can therefore be kept smaller than in the case of unhinderedlight spreading. Moreover, it is possible to image a plurality ofdifferent light sources in a spatially resolved way onto distinct targetlocations without (or with reduced) crosstalk.

The first DOE and/or the second DOE may particularly be one-dimensionalgratings which by definition have a (periodic) structure in a firstdirection and a constant form in a second, perpendicular direction.Alternatively, the first and/or second DOE may be two-dimensionalgratings with (periodical) structures in two perpendicular directions.

Every diffractive optical element shows a characteristic intensitypattern of the diffracted light when it is illuminated with a plane waveof light, wherein said pattern is determined by the design parameters ofthe DOE (for example by the width and distance of the slits in amulti-slit grating). The intensity pattern can be described by thediffractive orders which classify the effects of constructive ordestructive interference taking place behind the DOE. In the case of theoptical system described here, the first and/or the second DOE arepreferably designed such that more than 80%, most preferably more than95% of the intensity of diffracted light leaving the DOE is contained inone diffractive order. It is therefore possible to concentrate on thesignal light in said order, i.e. to design the optical system such thatlight of this order is directed to the target location while lightpropagating in other diffractive orders may be neglected.

The optical system may be used for many different tasks. For animportant class of applications, the optical system may comprise asample chamber adjacent to the substrate mentioned above, wherein aluminescent sample material can be provided in said sample chamber. Inthis case the signal light from the luminescent (e.g. fluorescent)sample material can be collected with a high efficiency and mapped in aspatially resolved way onto the target location.

According to another development of the invention, the optical systemcomprises an array of detector elements disposed at the target location(i.e. disposed such that the target location lies in the array). Thearray of detector elements may particularly be the sensitive area of aCCD camera. With an array of detector elements it is possible todistinguish signal light from different light spots because the imagesof the spots are mapped on different detector elements of the array.

The invention further comprises a method for mapping signal light fromat least one light source onto a target location corresponding to theimage of the light source, comprising the following steps:

-   -   a) Diffracting signal light a first time, for example with a        first DOE.    -   b) Imaging said diffracted signal light to a target location        according to the principles of geometrical optics.    -   c) Diffracting said focused signal light for a second time (e.g.        with a second DOE) such that (all or a part of) it interferes        constructively at the target location, which results in a spot        at said location (i.e. an image of the light source).

The method may optionally comprise the following further steps:

-   -   Coupling signal light from the light source into a substrate,        said substrate having a backside through which the signal light        may at least partially leave it.    -   Coupling so-called SC-modes of the signal light out of the        substrate by the first diffraction process, wherein the        “SC-modes” by definition comprise light that would be totally        internally reflected at the backside of the substrate if no        diffraction would take place.

The method comprises in general form the steps that can be executed withan optical system of the kind described above. Therefore, reference ismade to the preceding description for more information on the details,advantages and improvements of that method.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

In the following the invention is described by way of example with thehelp of the accompanying drawings in which:

FIG. 1 shows an optical system according to the present invention withsinusoidal gratings;

FIG. 2 shows the principle of diffraction at a sinusoidal grating forthe transmitted diffraction orders;

FIG. 3 illustrates in a perspective view the intensity distribution oflight in the SC-modes diffracted by a one-dimensional sinusoidalgrating;

FIG. 4 illustrates a setup used to calculate the intensity distributionof the optical system;

FIG. 5 shows an investigation apparatus for the investigation ofluminescent material with an optical system according to the presentinvention.

FIG. 1 shows schematically the setup of an optical system according to apreferred embodiment of the present invention. A light source 1 isdisposed adjacent to a transparent substrate 11, which may for instancebe a flat glass plate. The light source 1 may be a spot of samplematerial to be investigated that emits signal light (e.g. fluorescence).It should be noted, however, that such an investigation system is onlyone example for the application of the present invention.

A detailed analysis of the propagation of signal light emitted byluminescent particles 1 through a glass substrate 11 can be found in theWO 02/059583 A1 which is incorporated into the present application byreference. According to this analysis a considerable part of the signallight emitted by the light source 1 is contained in the SC-modes, whichare indicated by the representative rays L₁, L₂ and which comprise thesignal light that reaches the backside 10 of the glass substrate 11under angles of total internal reflection (provided that the mediumcontacting the backside 10 has a lower index than the glass substrate,for example if it is air). Thus the light of the SC-modes is normallycaptured inside the glass substrate 11 (leaving it perhaps in sidewarddirection) and lost for detection purposes.

To make the light of the SC-modes usable, a first diffractive opticalelement 21 is disposed at the backside 10 of the glass substrate 11 thatcouples out the light L₁, L₂ contained in the SC-modes. In theembodiment of FIG. 1, said diffractive optical element is realized by a(one-dimensional) sinusoidal grating 21 which may for example be etchedinto the glass surface.

The working principle of a sinusoidal grating 21 is schematically shownin FIG. 2. When irradiated with a planar light wave L₀, said light isdiffracted by the grating 21 and propagates behind the grating 21 with acertain intensity pattern in all directions. Different directions ofthis pattern are characterized by the order −N, . . . −m, . . . −1, +1,. . . +m, . . . +N of interference and carry different lightintensities. For the purpose of the present invention, the grating 21(or any other grating used instead of it) is designed such thatsubstantially all of the light will be contained in one dominatingdiffractive order, for example the m=2nd order (indicated by fat arrowsin FIG. 2). Moreover, the reflection of the gratings will preferably bemade as small as possible.

In the optical system shown in FIG. 1, a sinusoidal grating 21 of thekind shown in FIG. 2 is irradiated by the SC-modes L₁, L₂ under theangle of the dominating order of this grating. In this case the light ofthe SC-mode L₁ is coupled out of the glass substrate 11 and propagatesbehind the substrate mainly in a ray bundle L₁₁ (corresponding to order−m in FIG. 2) and a ray bundle L₁₂ (corresponding to the incident lightL₀ in FIG. 2). In a similar way the second light beam L₂ propagates inray bundles L₂₁, L₂₂ behind the glass substrate 11.

In the arrangement shown in FIG. 1, all of the signal light emitted bythe light source 1 and coupled out of the glass substrate 11 is capturedby a focusing lens 31 and converges behind said lens towards an imageplane P (without the presence of grating 41 on a substrate identical tosubstrate 11, the image plane can be at a different position). Withoutthe sinusoidal grating 41 and lens 31, the diffracted orders L₁₁, L₁₂,L₂₁, L₂₂ would not be diffracted into the orders L₁₃ and L₂₃ (which arethe reverse of rays L₁ and L₂ with the proper phase such that lightsource 1 is imaged as spot 51). This would result in ghost spots spreadacross the image plane P and lead to undesirable crosstalk if aplurality of light sources were present.

In order to prevent the aforementioned crosstalk, the optical system ofFIG. 1 comprises a second diffractive optical element DOE in the form ofa sinusoidal grating 41 that is disposed in a mirrored arrangement withrespect to the lens 31 and the first grating 21. Moreover, the secondsinusoidal grating 41 is preferably of the same type and dimensionalityas the first grating 21. The second grating 41 inverts effects of thefirst grating 21 on the optical path of signal light from the lightsource 1 (i.e. on the virtually prolonged path of ray bundles L₁, L₂),so that there are two light bundles L₁₃, L₂₃ behind the second grating41 which converge to the target location 51. Thus all signal lightemitted by the light source 1 is concentrated at one image spot, and aplurality of light sources can be mapped spatially resolved onto theimage plane P.

The function of an arrangement according to FIG. 1 relies on thefulfillment of the reciprocity principle and on the fact that all of thediffracted light L₁₁, L₁₂, L₂₁, L₂₂ is captured by the lens 31, i.e.that this lens has a sufficiently large NA. Due to the reciprocityprinciple (or principle of reversibility, cf. E. Hecht, “Optics,” 2ndedition, Addison-Wesley, Reading, Mass., chapter 4, 1987), for a givenconfiguration with a number of input rays (plane waves) and output (e.g.scattered, reflected, transmitted) rays, reversing the direction of allthe output rays results in the input rays now traveling in the reversedirection.

FIG. 3 shows in a perspective sketch the cone of light L₁, L₂ in theSC-modes emitted by the light source 1 and diffracted by aone-dimensional sinusoidal grating 21. As is illustrated by theintensity distribution above the grating 21, an amount of about 50% ofthe light intensity in the SC-modes is coupled out by the grating 21.With a two-dimensional sinusoidal grating, a larger angle range can beobtained to couple the light out, compared to a 1D grating. This wouldhowever lead to the generation of five spots, four of which would beghost spots.

FIG. 4 shows the principal setup of an optical system according to thepresent invention which allows to use the functionality of a firstgrating 21 without compromising for imaging quality, because the secondgrating 41 folds back the orders to the original image withoutgenerating ghost spots. The first grating 21 can e.g. be a gratingoutcoupler that frustrates total internal reflection, or it can be awavelength filter with pronounced transmission for a small wavelengthregime.

For a more detailed numerical analysis, the model system of FIG. 4 isassumed to consist of two identical gratings 21, 41 with a sinusoidalgroove in glass that are separated by the lens 31. The diffractionpattern of grating 21 is imaged 1-1 on grating 41. The arrows indicaterays of input light I, diffracted orders DO, diffracted orders imaged bylens IDA, imaged input II, and ghost image GI. Moreover, the followingparameters were assumed:

Refractive indexes: Glass, n=1.5; Air, n=1;

Grating: Period of 10 microns and grating depth of 250 nm.

Wavelength: 1 micron.

Polarization: TE

Input: Plane wave at normal incidence.

The diffraction efficiencies of the gratings were calculated using arigorous grating solver. As an approximation only the first 5diffraction orders (orders −2, −1, 0, 1, 2) of grating 21 were included.From the following Table 1, it follows that this is a reasonableapproximation.

TABLE 1 Order Diffraction Efficiency of first grating 4 1E−08 31.22E−06   2 0.000328 1 0.035778 0 0.887649 −1 0.035787 −2 0.000331 −31.25E−06   −4 1E−08

The total transmission of the first grating 21 is 96%. Thus 4% of thepower is in the reflected orders being in good agreement with theFresnel reflection on a glass/air interface for normal incident light(4%).

Using the 5 orders as input for the second grating 41, the optical powerin the orders in the glass layer of grating 41 were calculated, whereinthe order having the same angle as the input is denoted by “II” and theother orders are considered as ghost spots (GI). For a good image, theamount of power in II should be large compared to the power of GI.

Lens with NA=1:

Table 2 shows the fraction of input in orders behind grating 41classified into ghost images (GI) and image (II):

Class Order Fraction of input GI −2 0.00% GI −1 0.00% II 0 92.09% GI 10.00% GI 2 0.00%

From Table 2 it can be concluded that using a lens with NA=1 results ina virtually perfect image of the input beam, with no ghost images. Thetotal amount of the input power in the central spot (II) is slightlysmaller than the power resulting after two Fresnel reflections at aglass-air interface for normal incident light: 92.16%. This smalldifference can probably be attributed to the fact that not alldiffraction orders generated by grating 21 were included.

Lens with NA<1 that is Capable of Imaging First Three Orders: −1, 0, 1:

Table 2 shows the fraction of input in orders behind grating 41classified into ghost images (GI) and image (II):

Class Order Fraction of input GI −2 0.03% GI −1 0.00% II 0 91.94% GI 10.00% GI 2 0.03%

The fraction in first order ghost spots is still virtually zero; thisindicates that power in first order ghost spot is determined byinterference between

i) contribution that experienced first order diffraction by grating 21and fundamental order diffraction by grating 41, and ii) contributionthat experienced fundamental order diffraction by grating 21 and firstorder diffraction by grating 21.

Lens with NA<1 that is Incapable of Imaging Orders≠0:

Table 4 shows the fraction of input in orders behind grating 41classified into ghost images (GI) and image (II):

Class Order Fraction of input GI −2 0.03% GI −1 3.18% II 0 78.79% GI 13.18% GI 2 0.03%

Further reducing the NA of the lens to the point that it can only imagethe fundamental order results in an image similar to the image of asingle spot.

The simulations show that using two identical gratings where thediffraction pattern of the first grating is imaged on the secondgrating, it is possible to image the input of the first grating behindthe second grating without the presence of ghost spots provided that theNA of the imaging lens is sufficiently high in order to allow adjacentdiffraction orders to interfere. Based on this fact, one can for exampleuse the first diffraction grating as an outcoupling grating thatfrustrates total internal reflection, and by using the second gratingcollect each angle of the radiation into a single diffraction order oneachieves a virtually perfect image of fluorescent beads. Because theimage behind the 2nd grating is essentially perfect, one canimage/spatially resolve multiple beads using this method.

Because the principles described above work for a sinusoidal gratingirrespective of the grating period and because any grating shape can beexpressed as a sum of sinusoidal gratings, the described principles alsowork for any other grating shape. Thus blazed gratings could forinstance be used instead of the sinusoidal gratings 21, 41 in FIG. 1.

FIG. 5 shows the application of the optical system described above in aninvestigation apparatus. Said apparatus principally comprises amultiple-spot generator MSG 100 for the generation of an array of samplelight spots within the sample layer 302 of a biosensing unit 300(wherein only one representative sample light spot 1 of the array isshown in FIG. 5). The MSG 100 may for example be realized by a lightsource irradiating an array of apertures, thus producing an array ofsource light spots at the output of the MSG. Excitation light 504 fromone source light spot of the MSG 100 is focused (with optics not shownin FIG. 5) onto a sample light spot 1 in the sample layer 302 of asample chamber 303, wherein said sample chamber 303 is formed between aglass substrate 11 and a cover plate 304. The sample chamber 303contains a fluid with a fluorescent sample material, the fluorescence ofwhich is excited in the sample light spot 1 by the excitation light 504.A part of the stimulated fluorescence light then propagates into theglass substrate 11 as was discussed above for a general arrangement ofthis kind. According to the principles described above, fluorescentsignal light that propagates in the SC-modes is coupled out of thesubstrate 11 by a first diffractive optical element 21, for example asinusoidal grating. In FIG. 5, only the ray bundles L₁, L₂ of thefluorescence in the SC-modes are shown. The light of these bundles isreflected at the backside of a prism 207 of a dichroic beam splitter206, 207 which is designed such that the excitation light 504 may passunaffected while the fluorescence light is reflected. As was describedabove, a focusing lens 31 together with a second diffractive opticalelement 41 focus all the signal light emitted by a sample light spot 1onto a single image spot 51 in an array 50 of detector units (e.g. a CCDarray). Thus each of a plurality of sample light spots in the samplelayer 302 will be mapped to a different location (pixel) on the array 50allowing to measure them separately and with a high yield.

Finally it is pointed out that in the present application the term“comprising” does not exclude other elements or steps, that “a” or “an”does not exclude a plurality, and that a single processor or other unitmay fulfill the functions of several means. The invention resides ineach and every novel characteristic feature and each and everycombination of characteristic features. Moreover, reference signs in theclaims shall not be construed as limiting their scope.

1. An optical system with an imaging unit for mapping signal light from at least one light source on a target location, comprising: at least one first sinusoidal diffractive optical element (DOE) located in front of the imaging unit with respect to the optical path of the signal light; at least one second sinusoidal diffractive optical element (DOE) located behind the imaging unit with respect to the optical path of the signal light, wherein the at least one second sinusoidal DOE is configured so that the effect the first sinusoidal DOE has on the path of light rays passing therethrough is reversed by the second sinusoidal DOE; a substrate comprising a backside, wherein the first sinusoidal diffractive optical element DOE is located at the backside of the substrate and is adapted to couple out signal light of SC-modes, which comprise signal light that would otherwise be totally internally reflected at the backside; and a dichroic beam splitter disposed between the first sinusoidal diffractive optical element and the second sinusoidal optical element, and configured to reflect the SC-modes.
 2. The optical system of claim 1, wherein the first sinusoidal DOE and the second sinusoidal DOE are of identical design and/or that they are disposed in a mirrored arrangement with respect to the imaging unit.
 3. The optical system of claim 1, wherein the first DOE and/or the second DOE is a one-dimensional or a two-dimensional grating.
 4. The optical system of claim 1, wherein the first DOE and/or the second DOE are designed such that more than 80%, preferably more than 95% of the output intensity is contained in one diffractive order.
 5. The optical system according to claim 1, wherein the imaging unit comprises a lens, preferably a lens with a numerical aperture NA of more than 0.8, most preferably as large as the index of the medium surrounding the lens.
 6. The optical system according to claim 1, wherein an array of detector elements is disposed at the target location.
 7. The optical system according to claim 2, wherein it comprises a sample chamber adjacent to a substrate in which a luminescent sample material can be provided. 